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wheel size

Submitted by roger_jordan on Mon, 18/04/2016
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Commando

My Commando has an 18" rear wheel that I intend to change to 19".Can anyone tell me if this will change the gearing significantly enough for me to need the speedometer gearbox changing as well?

Submitted by richard_payne on Mon, 18/04/2016

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This is a proper Commando that we're talking about, is it ? They all had 19" wheels front and rear from new and I can't imagine that the modifier altered the speedo drive.

Rolling diameter of a 100/90-19" is not so very different from a 110/90-18" (789.6 revs per mile compared with 799.1) There will be a greater difference between individual units.

The Smiths SSM instruments are not bad but as with any magnetic clock, there is no fixed relationship between cable turns and needle position.

Submitted by michael_sullivan on Mon, 18/04/2016

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Hmmm...

Let me see if I did my sums correctly.

The circumfrence of an 18" rim is 56.52"

The circumfrence of a 19" rim is 59.66"

So: not considering the differences in height that the different tires might bring into the equations, for any given engine rpm, the rear wheel, and speed over the ground of the bikewill be 5.5% faster. Conversly, the speedo should read 5.5% slower.

Did I do that correctly? My brain hurts and I am going to have a beer.

Mike

Submitted by robert_tuck on Tue, 19/04/2016

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Will your legs still reach the ground?, will the stands hold the bike up?. Your speedo is likely to be more accurate ,do you really want to go that fast?. Only kidding!.

Submitted by david_evans on Tue, 19/04/2016

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100-90-18 Roadrider is 25.3 inches diameter

100-90-19 Roadrider is 26.4 inches diameter

Michael's maths didn't include the tyre, about 3.4 inches greater circumference on the 19" tyre.

a good way to get an accurate speed is to download "android speedo" onto your mobile phone and lash it to your handlebars. GPS speeds will be right on the numbers. Car speedos generally over read to prevent people speeding.

Submitted by roger_jordan on Tue, 19/04/2016

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The rear tyre is 110/90-18. (Roadrider)

I find Richard's reply interesting, as I expected there to be more of a difference in revolutions per mile. I may try David's android speedo experiment. I have thought that the bike was a little undergeared, but it may be that the clocks are not accurate.

Engine sprocket 26 teeth

Clutch sprocket 57 teeth

Gearbox sprocket 22 teeth

Rear sprocket 42 teeth

Submitted by keith_glassborow on Thu, 21/04/2016

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I have a similar tyre dia and circumference 110x90-18 BT45 (and have used a similar AM26 RR.) Rolling the bike and measuring rear tyre rolled distance with me sitting on bike gives revs/1000 in 4th of 18.6 on a new tyre. That's with an overall 4.114:1 (35/72 belt and 21/42 final). Your gearing would be about 18.3 (4.185:1). My speedo overreads by 10% low down but decreases error as speed rises! I think that is slippage on the aluminum cup (Veglia) or the tacho changing. I see just under 40 at 2000 rpm and 75 at 4000 rpm. 85 is just over 4500 rpm. I have only a slim idea of the tacho accuracy and that could be losing the speed fight as the revs rise. However it all matches with gearing at cruising speeds.

Submitted by roger_jordan on Wed, 04/05/2016

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I have checked the accuracy of the speedo by using my car satnav. I then wondered if the tachograph might be over reading. This was after several of your replies seemed to discount the wheel size as being the culprit. At the weekend I removed the tachograph stripped and cleaned it (not very dirty) eliminated the end play from the drive shaft and lubricated the cable. Have now taken a short test ride, and the instruments now seem to be agreeing with each other. I had found that end play of the drive shaft could result in over reading inaccuracies from a search on Google. So don't know what exactly was the cure, maybe one or a combination of the above.

Submitted by boo_cock on Wed, 04/05/2016

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Who cares? Veglia speedo's are crap anyway! Also consider, was the speedo drive changed when the 18" tyre was fitted?

Submitted by roger_jordan on Thu, 05/05/2016

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My concern was the fact that I believed the bike was under geared.

I initially thought this was down to having an 18" rear wheel, but replies I received had me thinking of why the speed shown was low for the revs shown.

It then transpires that the tacho was reading higher than the true(?) Revolutions of the engine.

Now that the speedo has been checked for accuracy, and work carried out on the tacho, the readings are more in line with what could be expected.

Submitted by grant_tiller on Sun, 08/05/2016

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18 inch rear wheel is a good choice, and something people do a lot because there is a much better choice of rubber out there.

With your Avon Roadrider 110/90 tyre, the rolling circumference is so close to the original Dunlop TT100 4.10x19 that would have been on the bike that your speedo won't be noticeably different.

There is 0.6% difference

There are also as many 'pro' arguments for the rideability of an 18 inch tyre as there are 'against' arguments from the rivet counters.

My advice? Stick with what you've got and enjoy it!

 


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